The ultimate objective of this research is to determine the causes of retinitis pigmentosa so that successful treatment of this blinding eye disease can be devised. This study proposes to develop an in vitro model of retinal pigment epithelium phagocytosis is the RCS rat animal model of retinitis pigmentosa. For reasons discussed rat retinitis pigmentosa is the present animal model of choice. The defect in this disease is hypothesized to be an inborn error of the retinal pigment epithelial (RPE) phagolysosomal system for ingesting and digesting retinal rod outer segments. This hypothesis will be tested in an in vitro RPE tissue culture system. Phagocytosis by diseased and control RPE of diseased and control rod outer segments will be measured. Lysosomal enzymes will be compared in diseased and control RPE cultures. The cyclic nucleotide control influences on this system will be investigated. An in vitro system is proposed because of the necessity of working on a scale applicable to the small rat eye but, more importantly, because an in vitro system may eventually be applicable to human tissue.